The present invention generally relates to radio frequency signal emitter geolocation systems and, more particularly, to a system combining multiple sensor modalities to improve the accuracy of using time difference of arrival (TDOA) and/or frequency difference of arrival (FDOA) techniques to determine the geolocation of a signal emitter.
The ability to determine the geolocation of a signal emitter (e.g., a mobile phone, a walkie-talkie, etc.) has many applications such as search/rescue operations, police/border protection, maritime patrol/rescue and military actions. TDOA and FDOA are known methods for using radio frequency (RF) signals measured at two or more separated receivers to determine the geolocation of a radio frequency signal emitter (hereinafter “signal emitter”). TDOA locates the signal emitter by exploiting the difference in the path length or range difference between the signal emitter and each of the receivers and the corresponding propagation time difference between the signal emitter and the receivers. FDOA estimates the location of the signal emitter based on the frequency difference observations from the receivers. It differs from TDOA in that the receivers must be in relative motion with respect to each other and the signal emitter. This relative motion results in different Doppler shift observations of the signal emitter at each of the receivers. When the locations and vector velocities of the receivers are known, the geolocation of the signal emitter can be estimated using the observed relative Doppler shifts between pairs of the receivers.
TDOA and FDOA are sometimes used together to improve location accuracy, and the resulting estimates are independent to an extent. When the signal emitter is stationary relative to the receivers, currently known TDOA/FDOA techniques can reliably determine the geolocation of the signal emitter. However, when the signal emitter is not stationary, currently known TDOA/FDOA location algorithms or techniques are very sensitive to signal emitter velocity induced errors. Therefore, determining the geolocation of a moving signal emitter by using currently known TDOA/FDOA techniques alone is less than satisfactory. Numerous attempts have been made to improve the accuracy of known TDOA/FDOA techniques in locating a moving signal emitter. However, there appear to be no currently known TDOA/FDOA techniques that can adequately handle the problem caused by a non-stationary signal emitter.
Therefore, it is desirable to develop a system and method that can accurately and reliably determine the geolocation of a non-stationary signal emitter. Furthermore, it is desirable to provide a system and method that can improve the accuracy of TDOA/FDOA techniques for determining the geolocation of a non-stationary signal emitter.